Chez Chesler

Many window fan have a thermostat that shuts off the fan after the room is cooler than a certain point. That's nice for turning the fan on in the evening and letting it run overnight, without making the room too cold by morning. But that doesn't do any good when it is hotter outside than inside. If somebody is in the room, moving around the slightly hotter air can make it more comfortable than less hot still air. But if the room is unoccupied, one would rather not run the fan until the early evening (or after the afternoon thunderstorm.) Window fans ought to come with a second thermostat, or thermocouples and a tiny amount of intelligence, so the fan runs only when two conditions are met: The room is too hot, and the outside air is cooler than the inside air. (Other modes would move air around, or intake fresh air, for occupied rooms; and vent hot inside air to the outside, for unoccupied upstairs rooms.)